The feed water supplied to steam generators in electricity power stations must be suitably heated and must contain a very small quantity of dissolved oxygen. Thus, it is common practice to use an apparatus called a deaerator which is fed with water extracted from the condenser via low pressure reheaters and which performs the following three functions in particular;
it deaerates and reheats the water, said function being provided by steam bled from the turbine; PA1 it stores the deaerated water in a large sized container, thereby enabling fluctuations in circuit throughput to be absorbed; and PA1 it feeds the feed pumps with sufficient head at any operating regime.
As a result, the apparatus is placed fairly high in the machine room.
There are two main implentations of deaerators in which deaeration is obtained by reheating the liquid to be deaerated.
When a volume of liquid is in equilibrium with its own vapor and with incondensable gases, the quantity of incondensables dissolved in the liquid is proportional to a coefficient of solubility for the incondensables in the liquid and to the partial pressure of the incondensables in the total gas phase (i.e. vapor and incondensables) which is in contact with the liquid. Thus, if the mass of liquid is at a temperature for which it saturated vapor pressure is exactly equal to the total pressure (vapor and incondensables) occupying the gas phase above the liquid, and if said total pressure remains constant, then the partial pressure of the incondensables will by zero and so the quantity of incondensables dissolved in the liquid will also be zero. In order to achieve this, it is clearly necessary for the total pressure to remain constant and for means to be provided to allow the incondensables to escape from the device enclosure. The mass of liquid must therefore be raised to the saturation temperature which corresponds to the pressure occupying the enclosure above the liquid.
In a first known implementation that applies this principle, the mass of water is contained in a single enclosure. An inlet is provided above the liquid surface for reheat steam which may be saturated or superheated, and which is free from incondensables, said steam being injected into the mass of water via dip pipes having holes at their bottom ends, with water to be deaerated being injected from above via a spray nozzle and with a incondensables being removed in the vicinity of said nozzle into an enclosure at a lower pressure than the pressure in the deaerator. See French Pat. No. 2,176,157, for example.
In a second known implementation, the apparatus comprises two separate enclosures, both of which are subjected to steam pressure, with one constituting the storage tank and with the other, upper enclosure, constituting the deaerator per se. These enclosures are connected to each other via pipe work for removing water and bringing steam into equilibrium. Incondensable-free reheat steam is applied to the upper enclosure and is conveyed to the inlet spray nozzles for the water to be deaerated. The assembly includes bubble (or scrubber) chambers and perforated trays. The incondensables are removed from the vicinity of the water inlet nozzles. The assembly also includes a device for distributing the reheated and deaerated water within the mass in the storage tank.
The main drawback of the first known implementation lies in the fact that more or less sudden drop in steam pressure causes the liquid to rise up the dip pipes into the manifolds feeding them, thereby reverse feeding the steam source. If the steam is bled from a turbine, the turbine may be severely damaged. Non-return valves are installed in order to avoid this danger, but they are not completely secure. In addition, the waer is heated by less than can be achieved by the second implementation, because of the steam head loss in the manifolds and the pipe work, and the head of water to be overcome. Several tenths of .degree.C. are thus lost, thereby reducing the efficiency of the intallation. Further, the manifolds and the pipework which are often fed with superheated steam are difficult to design and are easily damaged in operation.
In the second implementation, this drawback is avoided, however having two distinct enclosures with different functions gives rise to very considerable bulk and the weight is generally greater too, since both of the enclosures are designed to withstand the reheat steam pressure. Cost is high and on-site assembly is relatively complex since both the storage container made of one or more items and the deaeration enclosure must be conveyed separately to the site and then installed relatively high up by means of cranes.
Proposals have been made in French patent document FR-A-No. 2,573,320 for a deaerating device to be installed in a single envelope, and to comprise a series of perforated trays receiving the water to be deaerated and spreading the liquid which falls into a central box having perforated pipework passing therethrough and supplied with bled-off steam, with the box including overflows via which a mixture of water and steam escapes in the form of an emulsion.
This device is particularly advantageous when installed in a condenser well, and thus at low pressure (less than 100 mb), however when it is installed in a separate enclosure, the supply of water is relatively small or else the diameter and/or length of the device and thus its bulk must be considerably increased, thereby directly opposing the desired aim. Further, the assembly comprising the deaerating device is relatively expensive and complicated since it includes a plurality of trays, ducts having expansion diaphgrams, pouring spouts, . . . .
The object of the present invention is to provide a deaerator which, for a supply of water similar to that provided by a deaerator of the type having dip tubes, includes a deaeration system of the same type as the second above-mentioned implementation, thus avoiding having dip tubes and being as small in volume and as light in weight as possible for a deaeration system of this type, i.e. close to the corresponding values of a deaerator using dip tubes.